First off, thanks for including the condition :)

I think there's a bug in _BlocBuilderBaseState. If I initialize a BlocBuilder with a condition, previousState defaults to null even if the provided bloc has an existing state...so the listener built in _subscribe() necessarily throws a null error on the bloc's next state change (assuming you're using an operator on previousState).

I think this can be fixed by setting _previousState = widget.bloc.currentState in the initState() method...ie. same assignment as state.